Sheeting and chunking has been a problem in commercial polyolefin production reactors for many years. In gas phase reactors, the problem is generally characterized by the formation of solid masses of polymer on the walls or dome of the reactor. These solid masses of polymer (e.g., the sheets) eventually become dislodged and fall into the reaction section, where they may interfere with fluidization, block the product discharge port, plug the distributor plate, and force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event,” which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting, chunking and/or fouling,” while used synonymously herein, may describe different manifestations of similar problems, in each case which can lead to a reactor discontinuity event.
There are at least two distinct forms of sheeting that occur in gas phase reactors. The two forms (or types) of sheeting are described as wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical sections) of the reaction section. Dome sheets are formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor.
When sheeting occurs with Ziegler-Natta catalysts, it is generally wall sheeting. Dome sheeting may occur with Ziegler-Natta catalysts, but the occurrence is rare. However, with metallocene catalysts, sheeting may be wall sheeting and/or dome sheeting.
As a result of the reactor discontinuity problems caused by sheeting, various techniques have been proposed to improve reactor operability. For example, various supporting procedures or methods for producing a catalyst system with reduced tendencies for fouling and better operability have been discussed in U.S. Pat. No. 5,283,218. U.S. Pat. Nos. 5,332,706 and 5,473,028 disclose a particular technique for forming a catalyst by “incipient impregnation.” U.S. Pat. Nos. 5,427,991 and 5,643,847 disclose the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discloses polymer bound metallocene catalyst systems. U.S. Pat. No. 5,661,095 discloses supporting a metallocene catalyst on a copolymer of an olefin and an unsaturated silane. WO 97/06186 discloses removing inorganic and organic impurities after formation of the metallocene catalyst itself. WO 97/15602 discloses readily supportable metal complexes. WO 97/27224 discloses forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond.
Others have discussed different process modifications for improving reactor continuity with metallocene catalysts and conventional Ziegler-Natta catalysts. For example, WO 97/14721 discloses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor. U.S. Pat. No. 5,627,243 discloses a distributor plate for use in fluidized bed gas phase reactors. WO 96/08520 discloses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123 discloses using sound waves to reduce sheeting. U.S. Pat. No. 5,066,736 and EP-A1 0 549 252 disclose the introduction of an activity retarder to the reactor to reduce agglomerates. U.S. Pat. No. 5,610,244 discloses feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414 discloses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels. There are various other known methods for improving operability including coating the polymerization equipment, controlling the polymerization rate, particularly on start-up, reconfiguring the reactor design, and injecting various agents into the reactor.
Others have discussed injecting various agents into the reactor. The use of antistatic agents, for example, has been the subject of various publications. U.S. Pat. No. 7,205,363 and WO 2005/003184 disclose the use of certain antistatic agents with metallocene catalysts to improve reactor operability. EP-A1 0 453 116 discloses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates. U.S. Pat. No. 4,012,574 discloses adding a surface-active compound having a perfluorocarbon group to the reactor to reduce fouling. WO 96/11961 discloses an antistatic agent for reducing fouling and sheeting in a gas, slurry or liquid pool polymerization process as a component of a supported catalyst system. U.S. Pat. Nos. 5,034,480 and 5,034,481 disclose a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistatic agent to produce ultrahigh molecular weight ethylene polymers. For example, WO 97/46599 discloses the use of soluble metallocene catalysts in a gas phase process utilizing soluble metallocene catalysts that are fed into a lean zone in a polymerization reactor to produce stereoregular polymers. WO 97/46599 also discloses that the catalyst feedstream can contain antifoulants or antistatic agents such as ATMER 163 (commercially available from Croda, Edison, NJ USA). See also U.S. Pat. No. 7,205,363 and WO 2005/003184.
Antistatic agents have also historically been referred to as continuity additives, antifouling agents, or the like. For consistency in this disclosure, the term “polymerization additive” will generally be used hereinafter. One method of using polymerization additives that is known in the art may be referred to as a liquid slurry polymerization additive, which may be pumped directly into the reactor. To facilitate feed of a polymerization additive to a gas phase reactor, the polymerization additive is slurried in a hydrocarbon, mineral oil, or other liquid media. Batches of such liquid slurry polymerization additive may be formed and stored in an additive feed vessel for continuous supply to the polymerization reactor. To maintain the polymerization additive suspended in the liquid medium, use of an agitated vessel is often required, adding complexity and cost to the polymerization process.
The drawbacks with utilizing liquid slurry polymerization additives include the relatively complex nature of the liquid slurry preparation method, where extra drying steps may be needed for both liquid and polymerization additive to reduce moisture content. Also, the transportation of liquid slurry polymerization additive containers throughout the world may be costly. Furthermore, a mixing skid or agitated vessel, the latter typically a mechanically agitated vessel, may be required to ensure that homogenous liquid slurry polymerization additive is charged into the reactor, because the polymerization additive may settle out of the liquid over time.